Hydrogen storage of ammonia borane in ionic liquids

Currently, there is still a strong dependence on hydrocarbons as the energy demand is almost entirely satisfied with fossil fuels. A transition to new forms of energy is necessary and the use of hydrogen as an energy vector stands as an interesting alternative. But for an extensive use of hydrogen, many drawbacks still have to be overcome such as finding an efficient way of hydrogen storage for mobile devices. Gravimetric energy capacity of hydrogen largely exceeds that of gasoline but the volumetric capacity is still too low for practical applications.

A possible solution is the use of solid materials containing hydrogen which would desorb it through thermal treatment on-board. Many solid hydrides that can reversibly store hydrogen by absorption mechanisms have been tested to this aim and, in this context, ammonia borane (NH3BH3, AB) has shown to be very promising. It is a white crystalline inorganic solid, stable and safe to handle and it holds an outstanding gravimetric capacity of 19.6% of hydrogen releasing up to 2.2 equivalents of this gas by thermal decomposition. Besides the high hydrogen mass per unit mass of the material and to meet general requirements for a fuel, AB dehydrogenation should also exhibit ease of hydrogen evolution in terms of lower decomposition temperature, better kinetics and reversibility of the process to hydrogenated AB. Furthermore, neat AB thermal decomposition leads to desorption of other volatiles compounds such as ammonia, borazine or diborane which are detrimental and poisonous for the fuel cell to be fed. AB gravimetric capacity largely surpasses the value set by the U.S. Department of Energy (DOE) of 0.090 kgH2/kg for hydrogen storage materials thus allowing the use of additives, but AB volumetric capacity still has to be improved. Another important feature to consider is the solid nature of AB and the potential advantages derived from the use of a liquid fuel, easy to transport and which wouldn´t involve costly replacement of the existing systems.

To circumvent this problem, the use of ionic liquids as solvents has been studied and proved to bring improvement to AB performance in terms of rates and extent of H2 release, besides providing a stable liquid media due to their remarkable properties. Their low vapor pressure make them suitable to replace volatile organic solvents for dissolving AB and their stability to high temperature allows operating at fuel cells working conditions. It is commonly accepted that ionic liquids provide an inert reaction medium which stabilizes polar transition states and considerably fasten AB decomposition but H2 release rate and extend strongly depend on the used ionic liquid.